Lecture 5 Flashcards
What did Archibald Garrod discover a connection in in 1902?
What was his area of research, and what happens in this condition?
Connected inherited human disorders with Mendel’s law of inheritance.
Alkaptonuria - urine turns from yellow to brown to black after being exposed in air.
What did Garrod observe?
Urine of alkaptonuria contains large amounts of homogentisic acid.
Alkaptonuria inherited in a Mendelian fashion (autosomal recessive).
What was Garrod’s hypothesis?
Patients lack enzyme necessary for breaking down homogentisic acid.
Lack of enzyme due to defect in gene.
What has the missing enzyme in alkaptonuria been identified as?
Homogentisic acid oxidase.
What did Beedle and Tatum identify?
Series of auxotrophic mutants after irradiating Neurospora spores with X-rays.
What are auxotrophs?
Mutant strains that cannot synthesise particular molecule required or growth, e.g. arginine, therefore will only grow when supplied with that molecule.
5 steps in screening for auxotrophic mutants?
- Culture individual spores on complete medium.
- Transfer to minimal medium (MM) to identify possible auxotrophs.
- Test candidates for growth on MM supplemented with different classes of nutrients (vitamins/amino acids/nucleotides).
- Test candidates for growth on MM supplemented with individual amino acids (or vitamins or nucleotides).
- Identify amino acid allowing mutant to grow.
What did Srb and Horowitz identify?
Three classes of arginine auxotroph.
What are the 3 classes of arginine auxotroph?
Class I -
MM + ornithine.
MM + citrulline.
MM + arginine.
Class II -
MM + citrulline.
MM + arginine.
Class III -
MM + arginine.
What is the interpretation of the Srb-Horowitz experiment?
Class I must be deficient in enzyme A - creating ornithine from precursor.
Class II must be deficient in enzyme B - creating citrulline from ornithine.
Class III must be deficient in enzyme C - creating arginine from citrulline.
What is the one gene, one enzyme hypothesis, and who was it coined by?
What is wrong with this hypothesis?
Each enzyme in a pathway was controlled by one gene.
Beadle and Tatum.
Not all gene products are enzymes, so “One gene - one polypeptide” more apt.
What provided evidence for mRNA?
Pulse-chase experiments.
How do pulse-chase experiments work?
15 min label with radioactive uracil shows concentration in nucleus.
15 min label with radioactive uracil then 90 minute chase with non-radioactive uracil shows it’s scattered across cytoplasm.
2 steps in gene expression?
Transcription (4 base to 4 base).
Translation (4 base to 20 amino acid).
What is transcription?
Synthesis of an mRNA molecule using one strand of the DNA as a template.
